1. Field of the Invention
The present invention relates generally to flame-retarded fibers, and more particularly to flame-retarded, polyolefin-based fibers. The inventive flame-retarded fibers are comprised of a thermoplastic polyolefin and a ring halogenated vinyl aromatic grafted onto a polyolefin, and may also include a halogenated bisphenol derivative.
2. Description of the Prior Art
Polyolefin fibers--predominantly polyethylene and polypropylene--are high volume/low cost synthetics that are remarkable for their stain and abrasion resistance. As with all plastics, certain uses have required that the flammability of the polymer be reduced. When decreased flammability has been required, it has generally not been provided by the fiber itself, but has instead been provided by one of the other components in the fabricated article. In carpeting, for example, enough fire retardant can be loaded into the latex binder to provide a measure of protection for the polyolefin face fiber.
One reason why the fiber itself has not been formulated to contain fire retarding agents is that no fire retarding agent has been found which leaves the fiber with an acceptable balance of normal polyolefin properties. In addition, the presence of an additive often adversely affects the melt spinning operation. For example, hexabromocyclododecane ("HBCD") can be compounded into polypropylene and the blend melt extruded into fibers. To be economical, however, the fiber must be produced at as high a rate as possible. This requires that melt viscosity be reduced by raising the die temperature. Unfortunately, normal processing temperatures for polypropylene cause degradation of HBCD and impart an unacceptable brown color to the fiber.
Many attempts have been made to produce acceptable ignition resistant polyolefin fibers and fabrics. Much of this work has focused on topical applications in which flame retarding agents were coated onto the surface of fibers after the fibers had been produced. Permanence was not always assured, as often the active ingredient was merely dispersed in a polymeric binder which was "painted" onto the polyolefin fiber. The quality of the binder determined the longevity of the treatment.
More sophisticated topical methods utilized phosphorus- or halogen-based monomers which were applied to the fiber and crosslinked in place. While providing ignition resistance, these approaches change the feel or "hand" of the fabric and reduce a key property of the polyolefins--soil resistance.
An important limitation of topical applications is that they are restricted to use with spun fiber, or more commonly, with constructed fabrics. They cannot be applied to the resin prior to--or what would be the most efficient, during --the fiber spinning operation. Topical applications therefore require an additional and costly step in the production process.
Other attempts to provide flame retardancy employ an "additive" approach. Almost any non-volatile compound containing bromine or chlorine may be mixed into polyolefins to provide some measure of ignition resistance. None has been successfully commercialized due to problems with the strength, color, odor or toxicity of the fiber.
One problem with known flame retardant additives is the difficulty of effectively dispersing them into the molten polyolefin. Additives are generally powders which do not dry blend evenly with plastic pellets. Because this mixture is fed directly from the melt extruder to the spinning die, localized concentrations of undispersed flame retardant will occur. This will cause plugging of the spinnerette and filament breakage, requiring a shutdown of tile process. One solution is to pre-disperse the additives in a suitable thermoplastic resin to form concentrated plastic pellets containing the modifiers. This solution, of course, requires additional and costly steps in the production process.
To avoid the problems associated with additives, attempts have been made to chemically bond modifiers to the polyolefin molecule to provide ignition resistant fibers. In one process, elemental chlorine and bromine are attached to the surface molecules of polyolefin fibers and films to provide self-extinguishing behavior. Similarly, the grafting of vinylchloride or vinylidene chloride to the surface of polyolefin fibers in order to obtain diminished flammability is known. As with the topical applications, these grafting approaches suffered from the limitation that they could only be applied as post-treatments, requiring additional steps in the production process.
Polyolefin fibers containing non-halogenated styrene grafted to polypropylene have also been manufactured. In one method, polypropylene-g-styrene is co-spun with polypropylene to produce a highly crimped fiber with good tensile strength. Similarly, the grafting of styrene onto pre-formed polypropylene fibers is known. In one method, a halo styrene in the form of chloromethylstyrene is grafted to polypropylene fiber to produce a material with improved sticking temperature, color fastness, water retention, antistatic behavior and wool-like hand with uniform dyeability. None of these methods provides adequate flame retardancy while retaining reasonable textile properties. Also, these approaches are generally limited to post-treatment of the fiber and therefore include the previously-discussed disadvantages incumbent therewith.
Concerning bromostyrene graft copolymers specifically, U.S. Pat. No. 5,077,337 to Atwell et al. discloses graft copolymers represented by the formula: ##STR4## in which n is an integer &gt;1; P is a polyolefin; and S is a side chain grafted to the polypropylene and having monomeric units of the formula: ##STR5## in which x=1 to 4; R.sub.1 is H or CH.sub.3 ; and R.sub.2 is H or a C.sub.1-4 alkyl, and notes that such graft copolymers would be an improvement over inert additives in the production of flame retardant spun fibers because clogging of spinnerettes and equipment wear would be avoided.
A need therefore exists for a polyolefin fiber which melts into the base polyolefin under normal spinning conditions leaving no solid particles to plug spinnerettes. A need also exists for a polyolefin fiber which is thermally stable, non-topical, non-blooming, non-volatile, UV stable, dry blendable and spinnable into 2 to 3 dpf microfibers without extra compounding steps. Also, a need exists for such a fiber which further does not impart objectionable odors to the spun fiber or constructed fabrics made thereof, and which has essentially the same texture or hand as nonflame-retarded fibers. Finally, a need exists for such a fiber which is further insoluble in water and is evenly distributed throughout the fiber to make it extremely resistant to removal by normal laundering conditions. The present invention addresses these needs.